This invention relates generally to controls for hydraulic vehicle transmissions. More particularly, but not by way of limitation, this invention relates to an improved transmission control system that includes a valve for determining the sequence of actuation of certain clutches within the transmission.
Vehicles, such as front-end loaders used in the construction and mining industries, may weigh as much as 200,000 pounds. Such vehicles are subjected to frequent forward and reverse movements in order to accomplish the tasks for which they were designed. To make such vehicles efficient, it is necessary that the directional changes between forward and reverse movements be accomplished while the vehicle is in motion. Manifestly, sudden changes in direction of such large vehicles impose tremendous loads on the vehicle drive trains and especially on the transmission clutches.
Attempts to solve the transmission control problems are exemplified by U.S. Pat. Nos. 3,799,308 issued Mar. 26, 1974, to Edward R. Erisman; 3,468,194 issued Sept. 23, 1969, to Joachim Horsch, et al; 4,046,160 issued Sept. 6, 1977, to Joachim Horsch; and 4,135,610 issued Jan. 23, 1979, to Probir K. Chatterjea.
The transmissions in which the above mentioned controls are utilized include a directional clutch, that is, a clutch which drives the vehicle in forward or reverse and one or more speed or range clutches which determine the velocity of the vehicle. In order to absorb the tremendous forces developed upon reversal in the direction of travel of these large vehicles, one of the clutches must be designed to absorb the forces and heat generated. Generally, the clutch designed to absorb such loading is the first speed or range clutch. Accordingly, it is desired that this clutch be engaged after the directional clutch so that it can absorb the forces generated.
In addition, it is preferable to be able to completely reduce the pressure in the range clutch when it is not engaged to avoid damage that may be caused by partial engagement.
The object of this invention is to provide an improved transmission control system for fully engaging and disengaging the transmission clutches in the proper sequence to absorb the generated force and to avoid damage to such clutches.